1. Field of the Invention
The invention relates to a fuel cell provided with a cell module having a hollow electrolyte membrane.
2. Description of the Related Art
A fuel cell converts chemical energy directly into electrical energy by providing a fuel and an oxidant to two electrodes that are electrically connected and electrochemically oxidizing the fuel. As opposed to thermal power generation, fuel cells display good energy conversion efficiency because they are not subject to the Carnot cycle restriction. Proton-exchange membrane fuel cells (PEMFC) are fuel cells which use a solid polymer electrolyte membrane as the electrolyte. These fuel cells are advantageous in that they operate at low temperatures and can easily be made small, which makes them particularly attractive as mobile power sources as well as power sources for mobile objects.
In a typical proton-exchange membrane fuel cell, when hydrogen is supplied as the fuel, the reaction shown in expression (1) progresses at the anode.H2→2H++2e−:  Expression (1)
The electrons produced by expression (1) pass through an external circuit and do work at an external load, after which they arrive at the cathode. The protons produced by expression (1) move through the proton-exchange membrane fuel cell from the anode side to the cathode side in a hydrated state by electro-osmosis.
When oxygen is supplied as the oxidant, the reaction shown in expression (2) progresses at the cathode.2H++(½)O2+2e−→H2O:  Expression (2)
Water produced at the cathode primarily passes through a gas diffusion layer and is then discharged outside of the fuel cell. Thus, fuel cells are clean power generating devices which emit only water.
Conventionally, most of the proton-exchange membrane fuel cells developed have a fuel cell stack which is made up of a plurality of stacked flat single cells. These single cells are manufactured by providing an anode catalyst layer on one side of a planar solid polymer electrolyte membrane and a cathode catalyst layer on the other side of the membrane so as to form a planar membrane-electrode assembly (MEA). A gas diffusion layer is then provided on both sides of this membrane-electrode assembly. Lastly, membrane-electrode assembly is sandwiched between planar separators.
In order to improve the output density of the proton-exchange membrane fuel cell, a very thin proton-conducting polymer membrane is currently used as the solid polymer electrolyte membrane. The thickness of this membrane is usually 100 μm or less. Even if an even thinner electrolyte membrane is used to further improve output density, the single cell still cannot be made dramatically thinner than it currently is. Similarly, the catalyst layers, gas diffusion layers, and separator, and the like are being made thinner, but even if all of the members were made thinner, there is still a limit to how much the output density per unit volume can be improved.
Also, carbon material in sheet form, which is highly corrosive, is normally used for the separator. The carbon material itself is very costly. Moreover, because grooves for gas flow paths are usually micro-machined in the surface of the separator in order to allow the fuel gas and oxidant gas to spread substantially evenly over the entire surface of the planar membrane-electrode assemblies, the cost of the separators is extremely high which increases the manufacturing cost of the fuel cell.
In addition to the foregoing problems, many more problems exist. For example, securely sealing the area around the plurality of stacked single cells so that fuel gas and oxidant gas do not leak from the gas flow paths is technically difficult in flat single cells. Also, bending or deformation of the planar membrane-electrode assemblies may result in a decrease in power generation efficiency.
In recent years, proton-exchange membrane fuel cells have been developed in which power generation is basically based on a cell module that has an electrode provided on both the inner surface side and the outer surface side of a hollow electrolyte membrane (for example, JP(A) 9-223507, JP(A) 2002-124273, JP(A) 2002-158015, and JP(A) 2002-260685).
A fuel cell having this kind of hollow cell module normally does not require the use of a member corresponding to a separator that is used in a flat fuel cell. Further, because power is generated by supplying a different type of gas to the inner side than is supplied to the outer side, there is also no particular need to form gas flow paths. Accordingly, low manufacturing costs can be expected. Moreover, the cell module has a 3-dimensional shape, so the specific surface area with respect to the volume is larger than it is with a flat single cell, which means that an increase in power generation output density per volume is expected.
Normally in a fuel cell with a hollow cell module such as that described above, a fuel gas including hydrogen or an oxidant gas such as air is supplied to the hollow inner portion, and an oxidant gas such as air or a fuel gas including hydrogen, which will react with the gas supplied to the hollow inner portion, is supplied to the cell module outer portion. Many electrolyte membranes are proton conductive, such that protons move in a hydrated state by electro-osmosis through the solid polymer electrolyte membrane from the anode (fuel electrode) side to the cathode (oxidant electrode) side, as described with regard to Expression (1) above. Accordingly, when the hollow inner portion is made the anode and power is generated by supplying hydrogen thereto, there may be a lack of moisture at the anode and electrolyte membrane on the anode side. As a result, good power generation may no longer be able to be achieved.
JP(A) 9-223507 discloses a fuel cell in which a positive electrode and a negative electrode are formed on a portion of a polymer electrolyte hollow fiber as a power generating portion, and a humidifying portion is provided in a different location than the power generating portion of the polymer electrolyte hollow system. In this fuel cell, air is supplied as the oxidant to the outer surface of the polymer electrolyte hollow system of the power generating portion, deionized water is supplied to the outer surface of the polymer electrolyte hollow system of the humidifying portion, and hydrogen is supplied as the fuel to the inner surface of the polymer electrolyte hollow system. Because of the nature of the polymer electrolyte to allow only deionized water to permeate it, the hydrogen that passes through the inner surface of the polymer electrolyte hollow system is humidified by the humidifying portion. Supplying this humidified hydrogen to the power generating portion prevents the power generating portion (i.e., the anode) on the inner surface of the hollow system from becoming dry. With this technology, however, there is a portion of the surface of the polymer electrolyte hollow system that is not used as an electrode, which restricts the electrode area.
In view of the foregoing problems, this invention thus provides a fuel cell having a hollow cell module, which is provided with a humidifying mechanism to effectively prevent the cell module from becoming dry.